The present invention relates to lead-in cables for connecting seismic streamers to a towing vessel and for transmitting seismic data from the streamers to data processing equipment aboard the vessel and, more particularly, is concerned with a low-cost, small diameter conductive rope-like lead-in cable having improved performance characteristics.
Lead-in cables are used at the front end of the towed seismic streamer spread to connect each streamer to the towing vessel. Lead-in cables are also used by the military at the front end of the towed seismic arrays used for locating other vessels at sea. The lead-in cable carries electrical power to the streamers (or seismic arrays) and seismic data from the streamers (or arrays) to data processing equipment aboard the towing vessel. In either of these applications, the lead-in cable must withstand the mechanical forces generated by movement of the vessel and towed streamers (or arrays) through the seawater.
Lead-in cables are usually terminated at their vessel end inside each storage reel and at their sea end with mechanical terminations capable of transferring loads through the system. They are stored and deployed from both fixed and slewing winches. Therefore, the details of these terminations and arrangements vary from installation to installation.
Normally, current seismic cables are constructed as a central assembly containing electrical and/or optical components around which steel armor wires are helically wrapped to provide both mechanical protection from cutting or bending, etc. and tensile strength. In some instances, the overall tensile strength of these cables is in excess of 120,000 pounds force (lbf.) to cope with the loads they are expected to experience in service. Loads are generated as the result of drag on the streamers, vibration of the lead-in cable resulting from vortex shedding, motion transmitted from the deflectors or doors used to achieve the separation of streamers, and inertial effects within the towing arrangement.
During normal service when towing, lead-in cables tow reasonably close to the water surface. However, the cables may sink if, for example, the vessel stops or the cable is severed. For this reason, the sea end terminations are sealed to prevent the ingress of water into the termination and electrical inserts that would result in loss of electrical integrity.
FIG. 1 illustrates the cross-section of a conventional lead-in cable in present use. Such a cable typically contains copper or copper alloy power and seismic data conductors bundled or twisted together to form an inner core. As illustrated, the core is mechanically protected by a layer of helically wrapped steel armor wire and by a non-metallic outer jacket. Copper is typically used instead of steel for the electric conductors in seismic cables because the electrical resistivity of copper is approximately one-fortieth ({fraction (1/40)}th) to one-sixth (⅙th) that of steel. However, in comparison with steel, copper conductors elongate more easily and distort at lower elongation values, or under compressive loading. Therefore, to achieve the required tensile strength, a relatively high factor of safety must be used for present cables comprised of primarily copper conductors. The tensile strength factor of safety for a seismic cable is the ratio of breaking load to working load. A factor of safety of 4:1 is typically used in present cables with copper conductors. A large safety requires correspondingly large gauge and diameter cable components, which increase the cable weight, overall diameter, and cost in comparison with a cable designed to a lower safety factor.
More recently, electro-optic cables have been introduced to the seismic exploration fleet in which the main multiplexed data transmission lines have been replaced with fiber optic lines to reduce the weight, and, more importantly, the diameter of the lead-in cable. FIG. 2 illustrates the cross-section of a typical electro-optic lead-in cable. As seen, the use of fiber-optic transmission lines results in a significant reduction in overall cable diameter. However, the use of copper power cores in these cables still requires a relatively high tensile strength factor of safety to ensure against cable distortion and damage in seismic operations.
The use of a lower factor of safety would reduce the overall diameter, weight, and cost of a lead-in cable. In addition, a lighter weight and smaller diameter cable advantageously permits an increase of the separation distance between the streamers towed behind the vessel. A smaller diameter lead-in cable is also advantageous because problems associated with fluid drag and vortex shedding increase with cable diameter.
Consequently, a need still exists in the seismic exploration industry and in the navy fleet for a small diameter, lightweight lead-in cable. Preferably, such a lead-in cable will contain little or no copper and can therefore be designed to a lower tensile strength factor of safety than present lead-in cables. Such a lead-in cable will preferably also be simple and inexpensive to manufacture and have an extended life expectancy. Ideally, such a lead-in cable can be designed to incorporate either fiber optic or conventional seismic data conductors.
The present invention addresses the aforementioned needs. According to one aspect of the invention, a lead-in cable for connecting a seismic streamer or towed array to a towing vessel is provided. The lead-in cable comprises a first electrical conductor at the center core of the cable for carrying a first polarity of power to the streamer. A first layer of insulation surrounds the first electrical conductor. A second electrical conductor for carrying a second polarity of power to the streamer surrounds the first layer of insulation. A second layer of insulation surrounds the second electrical conductor. A layer of seismic data conductors for carrying seismic data signals from the streamer surrounds the second layer of insulation. A metallic protective layer surrounds the layer of seismic data conductors for providing cut resistance to the lead-in cable. The second electrical conductor, the first and second layers of insulation, the layer of seismic data conductors, and the metallic protective layer are all concentrically disposed about the longitudinal axis of the first electrical conductor at the center core of the lead-in cable.
In a further aspect of the invention, a non-metallic protective layer surrounds the metallic protective layer of the lead-in cable.
In a more specific aspect of the invention, the non-metallic protective layer surrounding the metallic protective layer comprises thermoplastic polymer.
In a further aspect of the invention, a third layer of insulation is disposed between the layer of seismic data conductors and the metallic protective layer.
According to another aspect of the invention, the seismic data conductors comprise fiber optic cables.
According to an alternative embodiment of the invention, the seismic data conductors comprise signal core conductors.
In a further aspect of the invention, the first and second electrical conductors for carrying a first and second polarity of power, respectively, to the streamer comprise a metal selected from the group consisting of steel, copper clad steel, titanium alloy, or other high strength metal other than copper.
In another aspect of the invention, the metallic protective layer for providing cut resistance to the lead-in cable comprises a metal selected from the group consisting of steel, aluminum, copperweld, or other high strength metal other than copper.
According to another embodiment of the invention, a lead-in cable for connecting a seismic streamer or towed array to a towing vessel is provided. The lead-in cable comprises a first electrical power conductor for carrying a first polarity of power to the streamer. A second electrical power conductor for carrying a second polarity of power to the streamer surrounds and contains the first power conductor. A plurality of seismic data conductors for carrying seismic data signals from the streamer surrounds and contains at least one of the first and second electrical power conductors. Means is provided for electrically insulating the first and second power conductors from one another and from the seismic data conductors. Means for providing cut resistance to the lead-in cable is also provided.
In a further aspect of the invention, the means for electrically insulating the first and second power conductors from one another and from the seismic data conductors comprises a first layer of electrical insulation disposed between the first and second electrical power conductors, and a second layer of electrical insulation disposed between the seismic data conductors and the adjacent power conductor.
In a more specific aspect of the invention, the electrical insulation layers comprise thermoplastic polymer.
In another aspect of the invention, the means for providing cut resistance to the lead-in cable comprises a metallic protective layer surrounding and containing the plurality of seismic data conductors.
A still further aspect of the invention includes means for providing corrosion resistance to the lead-in cable.
In a more specific aspect of the invention, the means for providing corrosion resistance to the lead-in cable comprises a non-metallic protective jacket surrounding and containing the means for providing cut resistance to the cable.
In a still more specific aspect of the invention, the non-metallic protective jacket comprises thermoplastic polymer.
In another aspect of the invention, a method is provided for making a lead-in cable for connecting a seismic streamer or towed array to a towing vessel. The method comprises placing a first layer of insulation over the surface of a first electrical conductor, placing a second electrical conductor over the surface of the first insulation layer, placing a second layer of insulation over the second electrical conductor, placing a layer of seismic data conductors over the surface of the second insulation layer; and providing cut resistance to the lead-in cable.
In a further aspect of the invention, the step of providing cut resistance to the lead-in cable comprises covering the seismic data conductors with a metallic protective layer.
In a more specific aspect of the invention, the metallic protective layer comprises a plurality of wires wound helically around the outer periphery of the seismic data conductors.
An alternative aspect of the invention includes the step of covering the metallic protective layer with a non-metallic protective jacket.
In a more specific aspect of the invention, the step of covering the metallic protective layer with a non-metallic protective jacket comprises extruding a thermoplastic polymer layer over the metallic protective layer.
In a further aspect of the invention, the first and second layers of insulation are placed over the first and second electrical conductors, respectively, by the process of extrusion.
In a still further aspect of the invention, the second electrical conductor comprises a plurality of wires, and is placed over the surface of the first insulation layer by winding the wires of the second electrical conductor helically around the first insulation layer.
In a still further aspect of the invention, the layer of seismic data conductors is placed over the surface of the second insulation layer by winding the seismic data conductors helically around the second insulation layer.
In a still further aspect of the invention, the method for making a lead-in cable further includes the step of placing a third layer of insulation over the layer of seismic data conductors before providing cut resistance to the lead-in cable.
According to another aspect of the invention, a method is provided for towing a seismic streamer or towed array from a floating vessel. The method comprises providing a pair of elongated structural members for securing the seismic streamer or towed array to the vessel, providing a first polarity of electrical power to the streamer through one of the pair of elongated structural members, providing a second polarity of electrical power to the streamer through the other one of the pair of elongated structural members, and providing seismic data conductors in association with the pair of elongated structural members for carrying seismic data signals from the streamer to the vessel.
In another aspect of the invention, the method includes electrically insulating the elongated structural members from one another and from the seismic data conductors.
In a further aspect of the invention, the method includes providing cut resistance to the pair of elongated structural members and the seismic data conductors.
In a more specific aspect of the invention, the step of providing cut resistance to the pair of elongated structural members and the seismic data conductors comprises providing a metallic protective layer over the outermost one of the structural members and the seismic data conductors.
An alternative embodiment of the invention includes providing corrosion resistance to the pair of elongated structural members and the seismic data conductors.
In a more specific aspect of the invention, the step of providing corrosion resistance to the pair of elongated structural members and the seismic data conductors comprises providing a non-metallic protective jacket over the metallic protective layer.